A scanning mirror for an optical scanner may be driven by an electrostatic drive type actuator (electrostatic actuator) having a high consistency with a CMOS process. The electrostatic actuator may be a counter electrode type drive system as shown in FIG. 34 or a comb-tooth electrode type drive system as shown in FIG. 35.
Referring to FIG. 34, the counter electrode type drive system includes a mirror (mass part) 100 coupled to a torsion beam 101, and a fixed side counter electrode 103 disposed to face a movable side counter electrode 102 of the mirror (mass part) 100.
Referring to FIG. 35, the comb-tooth electrode type drive system includes a mirror (mass part) 110 coupled to a torsion beam 111, and a fixed side comb-tooth electrode 113 disposed to face a movable side comb-tooth electrode 112 of the mirror (mass part) 110.
Single crystal silicon is the preferred material for constituting the mirror surface because of its associated flatness and smoothness. However, when single crystal silicon is used, formation of the counter electrode type drive system of FIG. 34 becomes complicated, and the comb-tooth electrode type of FIG. 35 is preferred.
In an electrostatic drive type microscanner using the comb-tooth electrode of FIG. 35, driving force is electrostatic attraction generated from a potential difference between the fixed side comb-tooth electrode 113 and the movable side comb-tooth electrode 112. A rotation force is achieved by providing a height difference between both of the electrodes. More particularly, a maximum displacement at this time corresponds to the electrode height difference.
However, referring to FIG. 36, there is a case where a structure is used in which a silicon oxide film 121, a thin film silicon layer 122, a silicon oxide film 123, and a metal film 124 are formed in sequence on a silicon substrate 120, and the metal film 124 is formed as an electrode. In this case, a movable side comb-tooth electrode is constructed with the thin film silicon layer 122, and a fixed side comb-tooth electrode is constructed with the metal film 124.
Here, the height difference between the movable side comb-tooth electrode 122 and the fixed side comb-tooth electrode 124 becomes as follows.
Referring to FIG. 37, the thickness of the thin film silicon layer 122 is designated by Ts, the thickness of the silicon oxide film 123 is designated by To, and the thickness of the metal film 124 is designated by Tm, the electrode height difference becomes {(Ts+Tm)/2}+To. Because the electrode height difference is limited by the film thickness Ts of the thin film silicon layer 122 (as stated above), a large displacement (scan angle) cannot be obtained.
More particularly, when the movable side comb-tooth electrode is the thin film silicon layer 122, and the fixed side comb-tooth electrode is the metal film 124 formed on the thin film silicon layer 122 through the oxide film 123, the height difference becomes the height difference between the center points of both the electrodes. Then, for example, even when the thickness Ts of the thin film silicon layer (mirror) 122 is made 10 μm, and the thicknesses To and Tm of the silicon oxide film 123 and the metal film 124 are made 1 μm, respectively, the electrode height difference becomes 6.5 μm. In the case where the size of the mirror part (thin film silicon layer) in length and breadth is made 1000 μm, the maximum displacement becomes tan−1(6.5/500)=0.74°, and the scan angle is limited to twice the value, that is, 1.5°.
On the other hand, although a large displacement can be obtained when resonance is used, in this case, the operation is limited to resonant driving at a resonant frequency.